As a method for strengthening ground by injecting a consolidating material into a ground to push earth and sand outward, and consequently, give rise to density increase, a method has heretofore been known which comprises injecting a non-fluid grouting material (mortar) with low slump or no substantial slump into a ground to develop solidified masses in the ground, thereby consolidating the ground [see Japanese Unexamined Patent Publication No. 108449/1994 (Heisei 6)].
However, since the above-mentioned method requires a large-sized apparatus, the method is unsuitable for reinforcement of a foundation of an architectural structure in an area crowded with architectural structures in which liquefaction prevention work or the like is effected or a place just below an architectural structure.
On the other hand, there has been a void filling method which comprises separately delivering a flowable mortar containing cement as a main ingredient and a plasticizer such as a water glass, an aluminum salt or the like to a place near an injection inlet, combining and mixing these at the place to form a plastic grout having such low flowability that slump is 3 cm or less, and injecting the plastic grout into a ground.
However, if such a plastic grout is pressure-injected into a soft ground, weak ground, poor ground, loose ground or the like, fracture grouting is likely to occur to result in escape of the grout beyond an intended area of the ground in a vein-like pattern. Accordingly, it is difficult to effect ground improvement by the method.
Further, a conventional cement-based mortar grouting fluid is in a slurry form and has flowability and solidifies by hydration reaction. When such a fluid mortar is injected into a ground, it splits the ground while being injected and is likely to spread away, and considerable bleeding is likely to occur to cause phase separation and precipitation of the material in the ground and the precipitate solidifies in a vein-like pattern.
For reducing bleeding, it meets the purpose to increase cement content. However, if cement content is increased, there is a problem that hardening of the cement occurs earlier and thus a large area cannot be grouted, or that the cement splits the ground and spreads away. Further, a method has been proposed which uses an aluminum salt, a water glass or the like as a plasticizer in a fluid cement-based mortar. Such a plastic grout is suitable for hollow grouting. However, when it is injected into a ground, it hardens rapidly in the ground due to its high viscosity to allow no further injection or it splits the ground and is likely to escape.
Moreover, a method for strengthening a ground has been proposed which comprises separately pumping a cement-based suspension and a plasticizer, combining them just before injection pipes to form a plastic grout with slump of less than 5 cm, for example about 3 cm, and injecting the plastic grout into a ground to compress the surrounding soil particles.
However, slump of less than 5 cm approximately corresponds to about 10 cm in terms of flow although this somewhat depends on constituent ingredients of the grout. In this case, the gelled material is in such a state that it makes no substantial movement even when vibration caused by drop is applied thereto. If such a plastic grout with low slump is injected into a ground, separation occurs between water phase (water) and solid phase (powdery matter) in the ground, and the grout further loses flowability by dewatering to rapidly harden. Accordingly, no substantial time period in which the grout retains plasticity can be obtained. In consequence, injection pressure increases. Due to this, no further injection is allowed, or the grout splits the ground in a vein-like pattern and escapes in unspecified directions. Accordingly, no substantial ground strengthening effect is obtained.